problems with no cats

[theredZ]

I have a 98 silverado with the 350 vortec. OBDII system. Just recently it threw codes for catalyst inefficiency, which is obvious, but it has also started running lean. I dont understand why the catalyst code was just thrown, and why it has started running lean. Could this be caused by a bad O2 sensor, or is it something bigger? My setup now is true dual exhaust with no cats, and glasspacks. It has been like this since I bought the truck so it would be hard to believe that not having cats is just now causing such problems.

[farmboy575]

I had the same setup put on my truck right after i bought it and it threw codes too. I never had a problem with it running lean though. I would say that there may be bad readings going from the o2 sensors. I dont have that setup anymore so I cant really tell you if mine would have done the same.

[Quyonmob]

O2 may be fouled from not reaching full light off temps.

Starting point would be replacing the front O2(s), and gettign a custom tune to delete the rear O2(s) or run O2 simulators.

[theredZ]

thanks for the replies. I believe it was the MAF throwing the codes running lean, and now it is running horrible. Gonna replace TPS and MAF tomorrow and hopefully that is all that is wrong with it.

[marcomontanez]

I've done a lot of research and running no cats will cause rear o2 sensors to give bad data to the pcm which in return tells front o2 sensor to adjust air/fuel mix and eventualy after constant automatic adjustment the air fuel mix will be way off and will drastically affect performance and mpg. Best thing to do is buy a set of high dollar rear o2 simulators that give good oxygen and temp reading to keep your air fuel mix stable. Or I've heard of people getting there pcm tuned by blackbear to remove rear sensors, supposed to be the better choice of the two.